I was having as number of issues with the JLCPCB tool.
See Assign LCSC number function not working KICAD 8.0.2 · Issue #474 · Bouni/kicad-jlcpcb-tools · GitHub
The 2 main symptoms were the inability to assign parts and the generated BOM was wrong.
I tried reverting to previous versions of the plugin and kicad but nothing changed.
In the end I discovered that the problems only show up when I use a mapped drive.
I have my work files mapped to drive I.
If I open the kicad files using
I:.…\file.kicad_pro
then I can’t assign parts or generate a correct BOM
Using C:\I.…\file.kicad_pro
everything is OK